An exhaust gas flow pipe constituting an automobile exhaust gas flow path member, and a heat shield cover therefor are demanded to have salt corrosion resistance in addition to heat resistance, and therefore a ferritic stainless steel excellent in heat resistance has been frequently used. Vibrations from an engine reach the exhaust gas flow pipe, and noises caused by the vibrations may become a problem. In recent years, members of an automobile are demanded to have light weights for improving the fuel efficiency. The reduction in thickness of the exhaust gas flow pipe for reducing the weight tends to further increase the noises due to the vibrations. Furthermore, the vibrations from the engine occurring in the heat shield cover may cause muffled sound, so as to be an offensive noise source in some cases. There is a demand of a heat resistant stainless steel material that is excellent in capability of suppressing vibrations and noises from an exhaust gas flow pipe. Furthermore, there is a large demand for improvement of the vibration damping capability of a ferritic stainless steel material not exclusively for the automobile heat resistant member.
The mechanisms attenuating vibration energy applied externally to a metal single material are classified into a eutectic type, a dislocation type, a ferromagnetic type, a composite type, and others. A steel material having a ferrite phase as the matrix (metal basis material) is a ferromagnetic material, and thus various vibration damping materials utilizing a ferromagnetic vibration damping mechanism have been proposed.
For example, PTL 1 shows an example, in which a vibration damping capability is imparted to a steel material containing Cr. There is described that Cr has a function of enhancing the vibration damping characteristics, and the effect of the addition thereof is increased up to 20.0% by weight (paragraph 0026). However, the Cr content of the specific examples shown as the examples is 3.08% at most.
PTL 2 shows a technique of imparting a vibration damping capability by using a steel material containing large amounts of Si and Co. It is taught that Cr has a significant effect of enhancing the magnetostrictive, but decreases the loss factor when the content thereof exceeds 9% (paragraph 0015).
PTL 3 describes a technique of imparting a vibration damping capability by controlling the crystal grain diameter, the maximum specific magnetic permeability, and the residual magnetic flux density without addition of alloy elements, such as Al, Si, and Cr, in large amounts. There is described that the crystal grain diameter is 300 μm or less in consideration of the surface roughening in processing (paragraph 0023).
PTL 4 describes that a vibration damping capability is imparted by using an iron alloy containing Cr and Ga in large amounts.